6 August 1998

Yes, It Does Always Rain On The Weekend

Back when chaos theory was in fashion, aficionados used to talk about butterflies flapping their wings over Peking and causing cyclones in Fiji. The study of apparently innocuous events and their effect on the weather has one again come to the fore with researchers from Arizona State University (ASU) finding some interesting correlation between pollution and rainfall.

The study, published in Nature, suggests that rain is most likely to occur along the US Atlantic coast on the weekend and the weather is most likely to be better on a Monday, Tuesday or Wednesday. The most obvious culprit is the "natural" cloud-seeding effect created by the massive drift of East Coast pollution, which also follows a well defined weekly cycle. Even Atlantic hurricanes may feel the punch of the workweek, according to the study.

ASU climatologists Randall Cerveny and Robert Balling, Jr. examined and compared three different data sets - daily carbon monoxide and ozone measurements from a Canadian monitoring station on Sable Island off the coast of Nova Scotia, daily satellite-derived rainfall data for the Atlantic Ocean, and databases of coastal Atlantic hurricane measurements. In each case, when the two ASU scientists examined the data by day of the week, they found significant differences between days, and similar patterns of variation, with pronounced differences between beginnings and the ends of weeks. All three sets of climate data revealed a seven-day cycle.

In examining precipitation in the Atlantic, they found no daily variation when looking at the ocean as a whole, but a pronounced sine-wave pattern of variation for just the coastal areas, with average daily precipitation rising on Thursday and into the weekend and then dipping from Sunday through the middle of the week. Balling notes that when the team analyzed satellite data grid cells for an area a little further away from the coast, they found the same pattern, time-shifted in accordance with the rate of pollution drift.

Though the study does not directly address causation, a comparable fluctuation in the levels of East Coast air pollution points to an obvious connection. The fact that coastal hurricane intensity data taken from 1945 to 1996 follow a similar pattern (rather than being statistically uniform for each day of the week, as one would expect), supports this hypothesis.

The grey, smelly cloud of pollution has a strange silver lining, however. While pollution makes for more rainy weekends, it also apparently reduces the intensity of hurricanes that hit over the weekend, such that weekend hurricanes tend to be much weaker than, say, Tuesday storms. "Hurricanes are the biggest storms that we have on this planet, in terms of energy and precipitation," noted Cerveny. "And what we've found is that we're having an impact on them. It's a little daunting, when you start to think about it."

"The fact that pollution can affect rainfall is actually well understood," said Balling. "We just had to look for the evidence in the right place. The hurricane data, though, surprised the heck out of me."

Though the study has interesting implications, what most surprised the ASU scientists was the fact that no other researcher had ever attempted to analyze these major data sets in such a basic way.

"Interestingly, no one had ever looked at this pollution data from a daily standpoint before and, curiously, nobody has bothered to look at seven-day cycles in the weather data," said Cerveny. "Oftentimes the most fundamental research is that way you say to yourself 'why didn't anybody look at this' When we were putting this together, we went through every journal we could find, saying 'somebody has got to have done this before!' Luckily for us, no one had."